The present invention relates to radar. More particularly, the present invention relates to a method and apparatus for identifying materials based on propagation characteristics of electromagnetic energy other than through air or free space.
The propagation characteristics of electromagnetic energy which includes radio frequency energy, commonly used in modern communication and radar systems, in various materials gives rise to behavior which is generally governed by Maxwell""s equations. In order to solve Maxwell""s equations in a relatively straightforward manner, a number of simplifying assumptions are usually made. These simplifying assumptions may typically include assuming that a transmission media or portion of a transmission path is characterized as a non-dispersive, isotropic, homogeneous dielectric. Further simplifying assumptions may include seeking a steady state solution with little consideration paid to transient phenomena associated with, for example, the interaction of electromagnetic waves with their surroundings. In most radar applications, simply receiving and processing a signal return to gain meaningful information with regard to a contact of interest or target in the face of potentially disruptive phenomenon such as scatter or clutter is the primary aim of the system.
Problems arise however, when more information is desired about a radar contact of interest or when the radar contact of interest is obscured behind radar absorptive or dispersive media. Prior art radar systems are generally responsive to target cross section of the first object encountered and, with certain exceptions such as, for example, imaging radar or interrogating radar, cannot provide additional information regarding the contact. Such prior art radar systems further are incapable of generating a useable return from absorptive or dispersive materials, as energy associated with a transmitted radar signal is either absorbed or scattered by such materials to the extent that the energy return is negligible.
While using the simplifying assumptions generally associated with the use of Maxwell""s equations to predict the propagation behavior of electromagnetic signals is more than adequate for most radar applications, additional transient phenomenon may be observed when the simplifying assumptions are discarded in favor of more detailed analysis. The signals associated with these transient events are known generally as xe2x80x98precursorsxe2x80x99, and were first described by Leon Brillouin and G Sommerfeld in 1914. At the time, they were not regarded as being significant, Brillouin thinking of them as xe2x80x98vanishingly smallxe2x80x99. The original development of knowledge in the area of precursors was related to research associated with the theory of relativity, and in proving or disproving Einstein""s hypothesis that nothing could travel faster than the speed of light in a vacuum. Despite their discovery, precursors have not been used in prior art radar systems.
Several prior art systems do exist, for example, for expanding the capabilities of conventional radar, and for the identification of certain objects, with some of these systems using frequencies generally regarded as below conventional radar frequencies. U.S. Pat. No. 4,408,156 to Veys, for example, describes a method and an apparatus for identifying sheet articles of non-conductive material which are marked for identification purposes. The method and apparatus of Veys includes producing an identification signal for a sheet article of non-conductive material by incorporating a small quantity of thin conductive fibers capable absorbing and reflecting microwave radiation energy. Veys, however, does not disclose the use of precursors.
U.S. Pat. No. 5,241,314 to Keeler et al. describes an image lidar transmitter downlink for guidance of an underwater vehicle. Keeler, at best, allows for maintaining communications for control of underwater vehicles however does not disclose the use of precursors.
U.S. Pat. No. 5,315,561 to Grossi describes a system for transmitting an electromagnetic signal under water. The system of Grossi uses a low-frequency signal to overcome the absorptive properties of salt water and receives a scattered reflected signal from anomalies below the surface of the water. The system of Grossi in using low-frequency signals gains the advantage of deeper penetration in terms of skin depths as compared to a transmitter using higher frequency signals. However, it should be noted that Grossi fails to make use of precursors.
U.S. Pat. No. 5,053,772 to Lamper et al. describes a radar system using a method for motion and range closure compensation. The system of Lamper however fails to disclose making use of precursors and further fails to disclose the ability to penetrate a lossy media.
Another system is described in U.S. Pat. No. 5,357,253 to Van Etten et al. in which low-frequency signals are used to provide deep subsurface penetration. It should be noted that the system of Van Etten departs little from the related art of seismic radiation to detect and map subsurface layers and buried objects. It should further be noted that the system of Van Etten further fails to disclose a use of precursors. Similarly, other systems exist for providing various enhanced capabilities for various purposes such as penetrating salt water, removing surface clutter, image processing, and the like, however, none disclose the use of precursors which allow for the penetration of lossy media.
One prior art system, based on U.S. Pat. No. 5,502,442 to Kohlberg, describes a method and apparatus for improving the signal to clutter ratio of an airborne earth penetrating radar. Kohlberg describes relying on a comparison between the dispersive and non-dispersive response of signals returned from a subsurface object to eliminate dispersive signals or clutter. However, Kohlberg does not disclose the use of precursors.
Finding actual solutions to behavioral models associated with practical applications involving precursors is quite difficult. Such solutions, however, were discovered by Dr. Kurt Oughstun and G Sherman in the mid 1970""s, and may be further described in the text entitled xe2x80x9cElectromagnetic Pulse Propagation in Causal Dielectricsxe2x80x9d, K. E. Oughstun and G. C. Sherman, Springer-Verlag, 1994, see, for example, Chapter 1 and Chapter 9. However, as noted, it is apparent that no prior art systems exist which make use of the precursor phenomenon.
While precursors exist and may be exploitable the difficulty posed by, for example, solving the characteristic equations associated with the transient precursor phenomena remains unaddressed.
It would be appreciated in the art therefore for a method and apparatus for allowing a radar to see farther into dispersive materials, and return signals with much lower attenuation therefrom. Such a system could provide a detectable radar signal return.
It would further be appreciated in the art for a method and apparatus which would provide, for example, the ability to make practical use of the precursor phenomena by identifying the presence of objects obscured by dispersive material.
It would still further be appreciated in the art for a method and apparatus which would provide, for example, the ability to make practical use of the precursor phenomena by identifying materials associated with unobscured objects or by identifying materials associated with objects obscured by dispersive material.
To make use of the precursor phenomena, a method and apparatus for providing a precursor based radar is described.
Thus, in accordance with one exemplary embodiment, signal processing may be performed to identify an object in an environment, including receiving a precursor associated with an electromagnetic wave interacting with the object. Various properties associated with the object may be identified using one or more characteristics associated with the received precursor. Accordingly, the electromagnetic wave may be transmitted with a characteristic such as, for example, a sharp rise time, so as to generate the precursor when the electromagnetic wave interacts with the object. Such a characteristic may be generated using a circuit which includes the use of capacitive discharge and a semiconductor device such as a Drift Step Recovery Diode.
In an alternative exemplary embodiment in accordance with the present invention, an electromagnetic wave having the desired characteristics may be generated using a microwave diode switch where the signal may then be amplified through, for example, a broadband semiconductor amplifier or a traveling wave tube amplifier.
In still another alternative exemplary embodiment in accordance with the present invention the characteristic of the electromagnetic wave may include at least one phase reversal which may be achieved by dividing the electromagnetic wave and producing a divided signal and then phase modulating the original electromagnetic wave with the divided signal to generate the phase reversal. Phase reversals, like sharp rise time pulses, ARE be capable of generating precursors when such an electromagnetic wave having phase reversals interacts with the object.
In still another exemplary embodiment in accordance with the present invention, one or more channels may be established corresponding to one or more possible characteristics associated with the precursor such as, for example, precursor spectra. Each of the one or more channels may be associated with a corresponding one of one or more possible material properties associated with the object. The corresponding precursor spectra associated with the possible material properties may include, for example, water generated precursor spectra, radar absorptive material generated precursor spectra, and metallic oxides RUST generated precursor spectra.
In still another alternative exemplary embodiment in accordance with the present invention, a color display may present an image of the object with the material properties associated with the object indicated by displaying each of the material properties with a corresponding color. Colors associated with the material properties of the object may be displayed in proportion to respective values associated with one or more received precursor spectra.